1. Field Of The Invention
The present invention is concerned with catalysts useful for the treatment of gases to reduce contaminants contained therein. More specifically, the present invention is concerned with improved catalysts which may function as catalysts of the type generally referred to as "three-way conversion" or "TWC" catalysts. TWC catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing both oxidation and reduction reactions, such as the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. Such catalysts find utility in a number of fields, including the treatment of the exhaust gases from internal combustion engines, such as automobile and other gasoline-fueled engines.
2. Background and Related Art
General Background
In order to meet governmental emissions standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants in vehicle and other engine exhaust gases, so-called catalytic converters containing suitable catalysts are emplaced in the exhaust gas line of internal combustion engines to promote the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") and the reduction of nitrogen oxides ("NO.sub.x ") in the exhaust gas. Two separate catalyst members or beds can be used in series, the first to promote reduction of NO.sub.x and the second to promote oxidation of HC and CO, with optional oxygen (air) introduction between the beds. Alternatively, a single bed TWC catalyst, which substantially simultaneously promotes both oxidation and reduction as described above, may be used, provided that the air-to-fuel weight ratio of the engine whose exhaust is being treated is held close to the stoichiometric ratio. For the foregoing purpose, catalysts comprising one or more platinum group metals and, optionally, base metal oxides distended upon a high surface area, refractory oxide support are well known in the art. The support may comprise a high surface area alumina coating carried on any suitable carrier such as a refractory ceramic or metal honeycomb structure, as well known in the art. For example, see C. D. Keith et al U.S. Pat. No. 4,552,732. Such high surface area alumina materials, generally referred to in the art as "gamma alumina" or "activated alumina", typically exhibit a BET surface area in excess of 60 square meters per gram ("m.sup.2 /g"), often up to about 200 m.sup.2 /g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases. It is a known expedient in the art to stabilize such activated alumina supports against thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or, most usually, rare earth metal oxides, for example, ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith et al U.S. Pat. No. 4,171,288.
It is also known that bulk ceria may serve as a support for platinum group metal catalytic components and that the ceria may be stabilized against thermal degradation by impregnation with a solution of an aluminum compound, followed by calcination. For example, see U.S. Pat. No. 4,714,694 of C. Z.. Wan et al, which discloses aluminum-stabilized bulk ceria, optionally combined with an activated alumina, to serve as a refractory oxide support for platinum group metal components impregnated thereon. See also U.S. Pat. No. 4,708,946 of Ohata et al, which discloses a three-way conversion catalyst comprising an alumina-modified cerium oxide material and at least one precious metal selected from the group consisting of platinum, palladium and rhodium, together with activated alumina.
The art shows an awareness that although particulate cerium oxide (ceria) provides an excellent refractory oxide support for the other platinum group metals, a deleterious interaction occurs between rhodium and ceria under certain conditions of use of the catalyst. When used as a support for platinum, for example, the ceria enables the attainment of highly dispersed, small crystallites of platinum on the ceria support material and the use of bulk ceria as a catalyst support for platinum group metal catalysts other than rhodium, is disclosed in U.S. Pat. 4,727,052 of C. Z. Wan et al. As described at column 5, lines 1-36 of this patent, rhodium tends to react with rare earth metal oxides, including ceria, especially under high temperature conditions, which has a deleterious effect on catalyst activity. In this regard, see also U.S. Pat. No. 4,678,770 of C. Z. Wan et al, the disclosure of which is hereby incorporated herein.